1. Field of the Invention
The present invention relates to a hexagonal ferrite suitably used as a permanent magnet material, such as a motor for an automobile, and particularly relates to a magnet material containing a hexagonal magnetoplumbite ferrite, and a process for producing the same.
2. Description of the Background
As an oxide permanent magnet material, a strontium (Sr) ferrite and a barium (Ba) ferrite, which are of a magnetoplumbite (M type) hexagonal structure, are mainly used, and they are produced as a sintered magnet and a bonded magnet.
Among the various characteristics of a magnet, residual magnetic flux density (Br) and intrinsic coercive force (HcJ) are particularly important.
Br is determined by the density of the magnet, the degree of orientation of the magnet, and the saturation magnetization (4.pi.Is) determined by the crystal structure.
Br is expressed by the following equation: EQU Br=4.pi.Is.times.(degree of orientation).times.(density)
The Sr ferrite and the Ba ferrite of M type has a 4.pi.Is value of about 4.65 kG. The density and the degree of orientation each is about 98% at most in the sintered magnet, which provides the highest values. Therefore, Br of these magnets is limited to about 4.46 kG at most, and it has been practically impossible to provide a high Br value of 4.5 kG or more.
The present inventors previously discovered that the addition of appropriate amounts of La and Zn in an M type ferrite raises the 4.pi.Is value thereof by about 200 G at most, and a Br value of 4.5 kG or more can be obtained, as described in U.S. patent application Ser. No. 08/672,848. In this case, however, since the anisotropic magnetic field (H.sub.A), which will be described herein below, is decreased, it is difficult to obtain a Br value of 4.5 kG or more and an HcJ of 3.5 kOe or more at the same time.
HcJ is in proportion to the product (H.sub.A.times.fc) of the anisotropic magnetic field (H.sub.A (=2K.sub.1 /Is)) and a single magnetic domain grain fraction (fc), in which K.sub.1 represents a crystal magnetic anisotropy constant, which is determined by the crystal structure as similar to Is. The M type Ba ferrite has K.sub.1 of 3.3.times.10.sup.6 erg/cm.sup.3, and the M type Sr ferrite has K.sub.1 of 3.5.times.10.sup.6 erg/cm.sup.3. It has been known that the M type Sr ferrite has the largest K.sub.1 value, but it has been difficult to further raise the K.sub.1 value.
On the other hand, in the case where ferrite particles are in a single magnetic domain condition, the maximum HcJ is expected because the magnetization must be rotated against the anisotropic magnetic field to reverse the magnetization. In order to make ferrite grains into single magnetic domain grains, the size of the ferrite particles must be smaller than the following critical diameter (dc) as expressed by the following equation: EQU dc=2(k.multidot.Tc.multidot.K.sub.1 /a).sup.1/2 /Is.sup.2
wherein k represents the Boltzman constant, Tc represents a Curie temperature, and a represents a distance between iron ions. For M type Sr ferrite, since dc is about 1 .mu.m, in is order to produce a sintered magnet it is necessary that the crystal grain size of the sintered magnet be controlled to 1 .mu.m or less. While it has been difficult to realize such a fine crystal grain and the high density and the high degree of orientation to provide a high Br at the same time, the present inventors previously proposed a new production process to demonstrate that superior characteristics that cannot be found in the art are obtained, as described in Japanese Patent Application Kokai No. 653064. In this process, however, the HcJ value becomes 4.0 kOe when the Br value is 4.4 kG, and therefore it has remained difficult to obtain a high HcJ of 4.5 kOe or more with maintaining a high Br of 4.4 kG or more at the same time.
In order to control crystal grain size of a sintered body to 1 .mu.m or less, it is necessary to make the powder size in the molding step 0.5 .mu.m or less considering the growth of the grains in the sintering step. The use of such fine particles causes a decrease in productivity due to increased molding time and increased crack generation on molding. Thus, it has remained difficult to realize high characteristics and high productivity at the same time.
It is known that the addition of Al.sub.2 O.sub.3 and Cr.sub.2 O.sub.3 is effective to obtain a high HcJ value. Notably, Al.sup.3+ and Cr.sup.3+ lead to an increased H.sub.A and suppress the grain growth by substituting for Fe.sup.3+ having an upward spin in the M type structure, so that a high HcJ value of 4.5 kOe or more is obtained. However, when the Is value is reduced, the Br value is considerably reduced since the sintered density is reduced. As a result, the composition exhibiting a maximum HcJ of 4.5 kOe can only provide a Br value of 4.2 kG.
A sintered magnet of the conventional anisotropic M type ferrite has a temperature dependency of HcJ of about +13 Oe/.degree. C. and a relatively high temperature coefficient of about from +0.3 to +0.5%/.degree. C., which sometimes bring about great reduction in HcJ on the low temperature side and thus demagnetization. In order to prevent such demagnetization, the HcJ value at room temperature must be a large value of about 5 kOe, and therefore it is substantially impossible to obtain a high Br value at the same time. Powder of an isotropic M type ferrite has a temperature dependency of HcJ of at least about +8 Oe/.degree. C., although it is superior to the anisotropic sintered magnet, and a temperature coefficient of+0.15%/.degree. C. Thus, it has remained difficult to further improve the temperature characteristics. A ferrite magnet is excellent in environmental resistance and is not expensive, hence, it is frequently used in a motor in various parts of an automobile. Since an automobile may be used under severe conditions including intense cold and heat, the motor is required to stably function under such severe conditions. However, a conventional ferrite magnet exhibits considerable deterioration in coercive force under low temperature conditions, as described above.
Even though ferrite magnets afford such characteristics, ferrite magnets having low squareness (Hk/HcJ) in the demagnetration curve exhibit low (BH)max and a deteriorated change with time.
Thus, a need exists for a magnet having a high degree of orientation, which is obtained by a production process using an aqueous solvent. This would afford advantages in productivity, and moreover, would not cause environmental contamination, as when organic solvents are used, whereby use of equipment for preventing contamination could be avoided.